Up until now, most wind turbines used a multiplier to increase the rotation speed of the shaft coupled to the generator. However, the current trend in the manufacture of wind turbines with increasingly higher power output is to suppress the multiplier in the power conversion system, with the object of reducing the number of components and maintenance needs. This implies low rotation speed on coinciding with that of the wind rotor (maximum speed between 10 rpm and 12 rpm for a 6 MW machine) and, in turn, very high wind turbine torque. These types of multiplierless generators are normally known as “direct drive.”
The dimensions of these types of electric generators (diameter and length) depend on the torque achieved, being preferable to increase the diameter in relation to the length in order to optimise the weight and cost of the active parts (copper and permanent magnets, as well as magnetic material). This implies that the dimensions of a direct drive electric generator are often much larger than those of conventional generators, which complicates road and/or railway transport and onsite assembly. The most frequent solution is the design of electric generators wherein the stator, rotor or both are modular.
Additionally, in order to increase the magnetic field and optimise the quantity of active materials, gap width must be as small as possible. At the same time, in order for the achieved torque to be constant, gap dimensions must be constant, i.e. they must not vary due to vibrations of the wind rotor and the mechanical shaft. Consequently, in order to guarantee minimum and constant gap width, one of the most frequent solutions in generator structure design has been to give it great rigidity, resulting in very high overall assembly weight.
In short, two of the main requirements which must be fulfilled by a direct drive generator are a modular design that will allow transport thereof by road and a structural design that will ensure constant gap width and moderate weight. In the state of the art there are different references to large-sized and modular generators which attempt to resolve these problems, some of which are mentioned briefly below.
EP 2063115 proposes the design of a generator having a modular stator and rotor wherein the modules of both comprise an active part for generating power (windings and steel in the case of the stator, magnets in the case of the rotor) and a structural part for preventing deformations and transmitting stator and rotor loads to four support elements disposed at the front and rear part, two for each of the stator and rotor (in FIG. 3, stator supports 26 and 27 and rotor supports 18 and 19). The dimensions thereof are such that the joins therebetween are disposed at a smaller generator shaft radius than that of the gap, in such a manner that both the nacelle and the bearing structure can be transported. However, assembly thereof in the wind turbine is not resolved by the proposed design.
U.S. Pat. No. 5,844,341, as in the case of EP 2063115, discloses a generator wherein the joins of both the stator and rotor modules are disposed at a distance smaller than or equal to the gap (FIG. 2 and FIG. 3). This patent also indicates an assembly process wherein fixation of the stator modules to the structure is carried out individually and allows adjustment of the distance from the rotor modules once these have been assembled, allowing adjustment of gap dimensions. This avoids a complicated alternative wherein the stator must be assembled in a single piece around the rotor, given the high attraction forces and small margin, and with the same width as that of the gap (page one of said patent). However, the stator of an electric generator must normally be of the distributed winding-type in order to achieve greater gap torque homogeneity. When the generator comprises a stator of this type, electrical connections must be made between the windings of the different modules. Additionally, with this solution the connections must be made at height, increasing the difficulty and therefore the time required for this type of operation.
U.S. Pat. No. 7,456,534 discloses an electric generator being considerably larger in diameter than in length. Said document indicates assembly of the generator stator prior to assembly of the rotor, subsequently assembling each rotor section individually on the stator. In a large-sized generator, said assembly must be carried out at the wind turbine installation site, due to transport restrictions. Despite not being clearly specified, there are two alternatives for said process. In the first alternative, both the stator and the rotor are assembled on the ground and the assembly is subsequently hoisted to the top of the tower. In the second alternative, the stator is assembled on the tower and the rotor sections are subsequently assembled one by one thereon. The main drawback of the aforementioned method, particularly in the case of magnet rotors, is the difficulty of the operation due to the forces of attraction between said magnets and the magnetic material of the stator, in this case laminated steel, added to the complexity of performing said process at height.
As mentioned earlier, another major challenge in a multi-megawatt wind turbine equipped with a large-sized electric generator is minimising gap width and ensuring that the dimensions do not vary substantially during operation without substantially increasing the rigidity of the electric generator structure, which would imply an increase in the weight and cost of the structural part thereof and, consequently, an increase in the rest of the structural parts of the wind turbine (frame, tower, etc.). DE 10255745 discloses a solution consisting of disposing bearings near the gap (FIG. 2). The main drawback is the existence of a single bearing for both the wind rotor and the electric generator rotor. This causes all the loads and vibrations to be transmitted to the stator structure which, therefore, must be dimensioned so as to support it, guaranteeing adequate dimensional gap tolerances, with the ensuing increase in material and cost.